There are two basic types of laser radar systems currently in use, one type utilizes the same optical system to transmit and detect the laser signal whilst the other type of system utilizes separate transmission and detection systems which are usually placed side by side with respect to each other.
These prior art co-axial systems utilize large telescopes to collimate the laser beam into a highly parallel beam. An example of such laser radar systems of this type are those currently in use to track the passive retro-reflector arrays which have been positioned on the surface of the moon by the Apollo missions of the United States and the Lunakhod missions of the Soviet Union. The reason that such large telescopes have to be used in this type of laser radar system arises because the pulsed ruby lasers used to generate the laser signal are far from optimum, producing a laser beam with a divergence far above that predicted theoretically. On the other hand, the overall efficiency of the ruby lunar ranging systems currently in use, i.e. about 10.sup.-16 % are so low that the light gathering capacity of a large telescope is of considerable, and perhaps decisive, advantage in such a laser radar system. On the other hand, with the continual improvement in lasers, the laser used as a beam generator in a laser radar transmitter will provide beams of increasing quality both regarding their divergence and power levels. These improvements allow the large telescopes to be dispensed with at least as far as the transmitter is concerned. Possibly, a much poorer optical quality detector telescope could be used to enhance the detection capability of a laser radar system which relied solely on the high quality of its output laser beam for its overall performance. On the other hand, with the increase in transmitted power in high quality output beams, the detector portion of a laser radar system is considerably simplified and sufficient signal can be detected by utilizing the transmitter optics as part of the optical system of the detector. Such a process of duplicating the use of high precision and expensive optical components for both the transmission and detection portions of an optical radar system significantly lowers the overall installation and operating costs of such a radar system. However, slowing the transmission and detector optics does give rise to considerable difficulties when the output pulse repetition rate of the laser radar system is increased to such a level that it becomes impossible to switch from transmission to detection paths and vice versa. This difficulty is overcome in the present invention by purposely wasting up to 50% of the output signal and up to 50% of the echo signal collected for detection.
Many of the prior art laser radar systems have the laser beam generator as an integral part of the moving portion of the laser tracking system. This is inadvisable due to the fact that a sensitive laser system should not be moved unnecessarily particularly if it consumes a relatively large amount of electrical power which in turn necessitates very heavy electrical cables from a fixed site power source, a process which inevitably limits both the motion and precision capabilities of any such laser radar system. A prior art laser radar system which incorporates many of the required characteristics of a versatile laser radar system is now under construction in Hawaii, at the Haleakala Lunar Laser Ranging Observatory. This laser radar system has been designed to track the lunar retro-reflectors so that the Earth-moon separation can be determined to better than 10 cms. It consists of a Neodymium doped yttrium aluminium-garnet laser pulse generator which emits a near diffraction limited laser beam output. This means that the output beam is sufficiently collimated so that the use of a large telescope is not required to provide a sufficiently intense photon flux density on the surface of the moon. In fact, after the 1.06 micron output of the laser transmitted has been partially converted to a more conveniently detectable second harmonic wavelength (0.53 microns) the beam is expanded in a relatively small telescope and directed onto the surface of a steerable mirror which directs the beam onto the correct location on the surface of the moon where a small portion of it can be reflected back via any one of the passive retro-reflector arrays to be detected by a conventional optical telescope-detector system at the sight station. However, despite the fact that the Haleakala lunar laser radar installation utilizes some of the latest techniques, not only are these highly restricted in their scope in this particular installation but they are utilized only in the transmission side, the detection side being quite conventional.